GNUInstallDirs: Update CMAKE_INSTALL_MANDIR for FreeBSD

FreeBSD has switched from PREFIX/man ro PREFIX/share/man
see https://cgit.freebsd.org/ports/tree/CHANGES entry 20200115.

Issue: #22883

Modules/GNUInstallDirs.cmake

@@ -326,7 +326,8 @@ else()
     "Info documentation (DATAROOTDIR/info)")
 endif()
 
-if(CMAKE_SYSTEM_NAME MATCHES "^(([^k].*)?BSD|DragonFly)$")
+# FreeBSD has switched to share/man in 2020
+if(CMAKE_SYSTEM_NAME MATCHES "^(([^k].*)?BSD|DragonFly)$" AND NOT CMAKE_SYSTEM_NAME MATCHES "^(FreeBSD)$")
   _GNUInstallDirs_cache_path_fallback(CMAKE_INSTALL_MANDIR "man"
     "Man documentation (man)")
 else()